Intervertebral body spacers have been in use for many years. Typically, these spacers are constructed from polymers, such as polyetherether ketone (PEEK) polymer, titanium, and stainless steels. By inserting these devices into a disc space, restoration of the disc space height is reestablished. The device removes pressure on nerve structures and eliminates nerve entrapment caused by the otherwise collapsed disc space. More recently developed devices, constructed of polymers or composites, provide an additional benefit of being radiolucent when viewed by xray techniques.
The most significant problem of the aforementioned conventional intervertebral body spacers is that the rigidity of the spacer does not allow for load sharing with a bone graft or bone substitute. Such grafts are disposed in the intervertebral space in order to fuse the vertebral surfaces that define the space together. The aforementioned prior art devices cannot fuse the vertebrae together alone, so bone, bone substitutes, and bone morphogenetic protein (BMP) type materials are used to provide a means of bone fixation. However, Wolff's law states that bone grows along lines of stress. For good fusion to occur, the implant spacer must distribute a load to the graft. Another problem with conventional spacers that are made from bioresorbable materials, such as poly-L-lactides (PLLA), is that these materials have limited strength and are designed to resorb completely away. With limited strength, these devices can fracture under the high loads of the spine.
In addition to the above, fusion in vivo is a variable process. It may happen quickly, slowly, or not at all due to a variety of other reasons characteristic of the intervertebral space. If fusion does not take place and the bioresorbable materials resorb, the space established during surgery closes and the preoperative painful condition returns.
While the aforementioned devices can be suitable for a particular purpose to which they address, they are not as suitable to providing a device that permits load sharing with the bone graft or bone substitute material while providing sufficiently rigid support of the spine during a healing process. In contradistinction, the present invention provides an apparatus primarily developed for the purpose of creating controlled load sharing while providing sufficiently rigid support of a spinal implant system and preventing full collapse of the disc space should a fusion fail to occur.